This application claims priority of provisional patent application Ser. No. 60/238,749 filed Oct. 6, 2000.
1. Field of the Invention
The present invention relates generally to the field of equipment for testing hearing. More particularly, the invention pertains to a hearing test device that is capable of presenting a variety of acoustical or vibratory stimuli and that can be easily calibrated.
2. Background
To perform a hearing test, it is necessary to present acoustical or vibratory stimuli at precisely controlled levels. Devices that perform this function, called audiometers, have been available for many years. Audiometers are optimized for presenting certain types of stimuli, such as pure tones and noise, which are used in traditional hearing tests. These stimuli are typically generated internally. All other stimuli must be provided by an outboard audio playback device, such as a tape player or CD player, which is supplied by the user and connected to the audiometer's external audio input.
Conventional audiometers are calibrated using a manual procedure. For each type of internally generated stimulus, a separate calibration step is required; for example, a separate step is required for each pure tone frequency. Each step requires presenting the stimulus at a specified nominal level, measuring the system output with a sound level meter or vibration meter, and adjusting a calibration control until the measured output matches the expected output. The external audio input is calibrated by presenting a stimulus from the outboard audio playback device and adjusting a calibration control in a similar fashion.
Audiometers must present stimuli over a range of levels in excess of 100 dB. Programmable attenuators are provided for controlling output level. Attenuators may be placed before or after the power amplification stage. When presenting stimuli at low levels, however, applying purely pre-amplification attenuation results in a poor signal-to-noise ratio. Therefore, audiometers must provide post-amplification attenuators. The amount of signal attenuation provided by a post-amplification attenuator depends on the transducer impedance. Therefore, audiometers are typically designed to accommodate transducers of a specified nominal impedance.
Newer hearing tests require presentation of alternative stimuli. One such newer hearing test is the Hearing In Noise Test (HINT). Originally developed to assess the benefit of hearing aids, HINT is useful for the measurement of functional hearing ability. HINT is a pre- recorded adaptive speech test that measures the Reception Threshold for Sentences (RTS) in quiet and in noise. Each of the 12 alternate forms of the test consists of a 20-sentence list. The sentences are short (5–7 syllables) and simple (first grade reading level).
As mentioned above, conventional audiometers provide only limited support for presenting alternative stimuli. In particular, when using alternative stimuli, stimulus presentation is not easily automated, stimulus level control may be inaccurate, and compensation for frequency-dependent transducer characteristics is not possible. More tests which use alternative stimuli are likely to be developed as audiological science progresses. Therefore, a useful hearing test device should provide full support for the use of arbitrary stimuli.
There are several drawbacks to the design of conventional audiometers implemented as described above:                There is no easy way to automatically control an outboard audio playback device, so presentation of externally provided stimuli cannot be automated.        Calibration is time-consuming and susceptible to operator error.        The system can only be calibrated for a specific externally provided stimulus. If a different stimulus is presented from the outboard audio playback device, the stimulus presentation level cannot be predicted.        While the calibration procedure for internally generated pure-tone stimuli effectively compensates for the frequency-dependent response of the transducer, no such compensation is possible for broad-band or externally provided stimuli.        The system is designed to be used with transducers of a specified nominal impedance. This reduces the flexibility of the system by preventing its use with transducers of different nominal impedance.        The system is designed with the assumption that the actual impedance of a transducer is equal to its nominal impedance and is constant across frequency. In practice, either of these assumptions may be false, with the result that the system may present incorrect stimulus levels.        